Water-cooled nuclear reactors, particularly pressurized-water nuclear reactors, comprise a vessel which is intended for containing the core of the nuclear reactor and which is connected to the reactor cooling circuit in which the cooling water circulates.
The wall of the reactor vessel which is in contact with the cooling fluid and which is exposed to the radiation emitted by the reactor core can be activated and contaminated after the reactor has been in operation for some time.
In the case of nuclear power stations which have reached the end of their life and require a complete shutdown, the solution adopted in the past has been to leave these power stations in their existing state and to allow the activity of the constituent materials of their components to decrease, in order subsequently to dismantle them under more satisfactory conditions than at the time of the shutdown, without the need to employ complex, remotely controlled equipment.
The number of power stations put out of industrial operation will increase appreciably in the future, and it is therefore necessary to consider dismantling these power stations in order to restore the site where they are installed to its original state.
The dismantling of the conventional part of the power station presents no particular problem, but, in contrast, the dismantling of the part of the power station constituting the actual nuclear reactor poses problems which are difficult to solve in view of the radioactive emissions of the constituent materials of the reactor components.
In particular, the vessel of water-cooled nuclear reactors, which contains the fuel assemblies and which is in contact with the cooling water of the reactor during its operation, is activated and contaminated where reactors which have reached the end of their life are concerned.
As regards pressurized-water nuclear reactors in operation at the present time, the reactor vessel takes the form of a body of generally cylindrical shape closed by domed bottoms, of large size and having a considerable wall thickness.
The vessel, which has a very high mass, is arranged within a vessel well made in a concrete structure which also delimits one or more pools located above the upper level of the vessel.
The vessel which contains not only the fuel assemblies but also various internal structures, is connected by means of connection pieces to pipelines of the primary circuit of the reactor.
The core assemblies and some components of the internal structures can be dismantled and removed from the vessel, in order to ensure their disposal and, if appropriate, their elimination at the time when the reactor is put out of operation.
Some components of the highly-activated internal structures of the reactor, such as the shroud of the core, may need to be kept inside the vessel so as to be cut under water (radiological protection). Their dismantling has to be carried out within the vessel and during the operations of dismantling the vessel itself.
To date, no process and apparatus is known which enables the vessel of a pressurized-water nuclear reactor to be dismantled under without the risk of radioactive contamination in the work zone, while at same time using machining and handling means of relatively simple design in order to carry out the fragmentary disposal and elimination of the material of the vessel.